1. Field of the Invention
The present invention relates generally to a vehicular window glass antenna system and, in particular, to a window glass antenna system in which directional repeatability and reception sensitivity are improved by attaching an antenna switching circuit in the form of a printed-circuit board directly onto a pane of vehicular window glass.
2. Description of the Related Art
Japanese Patent Laid-Open Publication No. HEI 6-318811 and No. HEI 7-245516, both assigned to the assignees of this application, disclose a window glass antenna system, as shown in FIG. 4 hereof. The window glass antenna system 41 comprises a plurality of antennas 43, 44 and a defogging-heater/antenna 45, which are disposed on a pane of window glass 42 for receiving AM/FM radio and TV broadcast bands. The antenna system 41 also includes a plurality of power terminals 53, 54, 55 disposed on the window glass pane 42 for supplying power to the antennas 43, 44 and heater/antenna 45, and an antenna switching circuit 49 disposed externally of the window glass pane 42. The antenna switching circuit 49 has input terminals 33, 34, 35 connected to the power terminals 53, 54, 55 via respective power cables 33a, 34a, 35a. The antennas 43, 44 and heater/antenna 45 are switched over by control signals transmitted through a control signal cable 47. Received signals are taken out via a coaxial cable 48.
The window glass antenna system is advantageous in that since received signals can be taken out not only in their independent forms but also in combined forms, different kinds of received signals, which are greater in number than the number of the provided antennas, are obtainable. The system also has the advantage that since it incorporates the antenna switching circuit for switching the antennas one over the other so that the antennas may be used exclusively as an AM/FM radio or TV broadcast band receiver, interference between the AM/FM and TV antennas can be avoided.
However, in the prior window antenna system, since the power cables 33a, 34a, 35a are used for the connection of the input terminals 33, 34, 35 of the antenna switching circuit with the antenna powering terminals 53, 54, 55, the power cables 33a, 34a, 35a form part of the antennas 43, 44, 45 during operation thereof, whereby stray capacitance resulting from the power cables varies depending on the mode of mounting thereof, causing the antenna impedance to vary. This may result in deterioration of the directional repeatability and reception sensitivity of the antennas, thus requiring an extra time for antenna tuning.
Additionally, recent automobiles are designed such that the least running and engine noises may come inside the automobiles so as to provide the occupants of the automobiles with optimal comfort. As a result, faint noises, which have not been cared at all in the past, are much cared now.
Consequently, there is a demand for the provision of a window glass antenna system with improved directional repeatability and reception sensitivity.